[Transparent Gateway][Doorlock] Presence of Nordic boards in documentation

(Doc revision 48056d5a)

In the BOM summary and in the Co-Processor section, Nordic boards are mentioned, even if they are dismissed from the project.
Also, the image shown below the BOM contains the setup with Nordic board instead of Arduino.

RTD links:
https://docs.oniroproject.org/projects/blueprints/transparent-gateway.html#hardware-setup-bill-of-materials
https://docs.oniroproject.org/projects/blueprints/cats.html#bill-of-materials
https://docs.oniroproject.org/projects/blueprints/transparent-gateway-details.html#build-and-flash-the-radio-co-processor-firmware
https://docs.oniroproject.org/projects/blueprints/cats.html#prepare-openthread-radio-co-processor-rcp